| | Introduction | Materials and methods | Results and discussion | Acknowledgements | Literature cited
Introduction
To ensure an adequate supply of feed many farmers grow barley continuously for several years, and often it is the same cultivar year-after-year. However, continuous production of the same resistant barley cultivar places substantial selection pressure on the scald pathogen, Rhynchosporium secalis [Oudem.] J.J. Davis. Research has shown that there is a tremendous amount of diversity in the scald pathogen and therefore potential for rapid changes in the prevalent genotypes of the pathogen in response to the barley cultivars being grown, with some scald races having the ability to attack either individually or in combination several known sources of resistance (Tekauz 1991; Xi et al. 2002, 2003). A recent study demonstrated that barley cultivar rotation can be a potential short-term strategy to help reduce leaf disease levels and maintain crop productivity for Alberta barley producers where crop rotation options are limited due to feed requirements or market factors (Turkington et al. 2005). One of the cultivars used in the study, Kasota has maintained a relatively high level of resistance to the scald pathogen over a number of sites and years in Alberta (Xi et al. 2003). However, in the cultivar rotation study, although not statistically significant, there was a tendency for greater scald severity when Kasota was grown continuously under barley cultivar monoculture (BCM) compared to its production under barley cultivar rotation or in rotation with triticale. Scald severity on Kasota was very low (<1.0%) in both 1998 and 1999, but had increased to 4.0% on the flag and 7.9% on the penultimate leaf by year 2000 in the BCM treatment. Rotating Kasota with other barley cultivars or triticale maintained scald levels at 1.1% or less on the flag and 1.9% or less on the penultimate leaves.
Single spore isolations were made from symptoms on Kasota leaves from the continuous Kasota treatment (CKT) in the rotation experiment (Turkington et al. 2005) and in the current study were used along with three other R. secalis pathotypes derived from specific barley cultivars. Preliminary growth chamber trials indicated that the CKT pathotype produced susceptible reactions on Kasota at the seedling stage. The objective of the current study was to evaluate the occurrence of more virulent scald pathotypes, their pathogenicity on a range of barley genotypes and registered cultivars under field conditions in 2004, and to identify effective sources of resistance in commercial cultivars and accessions. The current study will help to identify the occurrence of R. secalis pathotypes with the ability to overcome some of the most effective sources of resistance present in commercially available barley cultivars. Knowledge about variation in pathogenicity of R. secalis will also help to identify compatible barley cultivar combinations that can be used as part of short-term cultivar rotation strategies for disease management.
Materials and Methods
Thirty-three barley accessions and cultivars with major resistance genes were studied for their differential reactions to R. secalis (Table 1). The 4 scald pathotypes used were derived from single spores collected from infected leaf material from each of the barley cultivars Kasota, Seebe, CDC Earl and Harrington collected from several experimental sites. Fertilizer applied to the experimental area consisted of 202 kg/ha of 13-22-22-0, which was broadcast and incorporated into worked pea stubble. Hill plots, consisting of 8-10 seeds planted in a single hole on 0.5 m centers, were arranged in a 4-replicate RCBD for each pathotype. Each pathotype trial was seeded a minimum of 15 m from another. Weed control was accomplished 8 June, 2004 with a tank mix of Refine Extra® at 20 g/ha and Puma Super® at 360 mL/ha. Scald inoculum was prepared from 2-3 week old cultures grown on lima bean agar. Culture plates were scraped using reverse-osmosis water and a hand-held battery operated toothbrush. The spore concentration was adjusted to 105 spores/ml and 2.5 L of inoculum for each pathotype was applied as a fine mist using a compressed air sprayer on 14 June and 28 June, 2004. After spores were sprayed onto hills, the cut up agar from the scraped plates from each pathotype was uniformly placed on the respective hills. Hills were rated for percent leaf area diseased (PLAD) using a 0-9 scale (0 = no disease in the lower, middle and upper canopy, 9 = >50% leaf area diseased in lower, middle and upper canopy) on 5 August, 2004.
An analysis of variance of the data was conducted separately for each pathotype using the PROC MIXED procedure of SAS, with block as a random effect and cultivar as a fixed effect (Littel et al. 1996). For significant effects LSD values were derived to allow for further exploration of treatment differences. Treatment effects were declared significant at P ≤ 0.05. A rating index was also derived from the mean cultivar ratings for each pathotype. Absolute differences in ratings were then calculated among all pathotype combinations and then summed for each cultivar to derive the rating index. A rating index of 0 indicated that the rating for a cultivar remained the same among the pathotypes tested; however increasing index values indicated that ratings changed from one pathotype to the next. Spearman’s rank correlations were calculated among the mean cultivar ratings for each of the pathotypes and were declared significant at P<0.05. Hudson was included in the present study, but due to its winter growth habit comparable ratings were not possible and data for this cultivar were not included in the analyses.
Results and Discussion
Cultivar had a significant influence (P<0.0001) on the severity of scald for all pathotypes tested. The trend was for the Kasota pathotype to produce somewhat higher disease severity compared to the other three pathotypes (Table 1). Although not included in the analysis, no symptoms were detected on Hudson when inoculated with the Kasota, Seebe, or CDC Earl pathotypes, while a low level disease was found for the Harrington pathotype (data not shown). The CDC Earl pathotype produced intermediate disease severity, while the trend was for the Seebe and Harrington pathotypes to produce the lowest levels of disease. For the Kasota pathotype, only Seebe and Shyri had relatively low, “resistant-type” reactions, while the highest levels of disease were observed on CDC Dolly, Harrington, AC Stacey, Kasota, Mahigan, Niska, Ponoka, Trochu, and Vivar (Table 1). Among the accessions, Abyssinian, Turk, and Osiris had the lowest levels of disease, while Atlas and API/CM67-B//AGER had the highest level of scald. All other cultivars and accessions had intermediate levels of disease. Different results were observed for the Seebe pathotype. The lowest levels of scald were observed on Manny, Osiris, Gatillo-Bar, Atlas46, Turk, Abyssinian, Seebe, Mahigan, and Shyri, while the highest levels were observed on Harrington, Peregrine, Niska, and Vivar. All other cultivars and accessions had intermediate diseases levels. For the CDC Earl pathotype, Manny, Abyssinian, Osiris, Seebe, Shyri, Kasota, AC Stacey, Ponoka, and Atlas had the lowest disease levels, while CDC Earl, Niobe, Harrington, Jaeger, Vivar, Modoc, Trochu, Duke, Leduc, Peregrine, and Trebi had the highest. The remaining cultivars and accessions had intermediate levels of disease. For the Harrington pathotype the lowest levels of disease were observed on Manny, Seebe, Osiris, Abyssinian, AC Stacey, Kasota, Shyri, Ponoka, Kitchin, Gatillo-Bar, and Atlas46, while the highest levels were observed on CDC Earl, Harrington, Niobe, Duke, Jaeger, Peregrine, and Trebi. Intermediate scald levels were observed on the remaining cultivars and accessions.
Table 1. Mean scald severity for barley accessions and Alberta cultivars, scald pathotype field study, Lacombe, Alberta, 2004.
 | Pathotype and scald severity (0-9 scale) | |
| Accession/cultivar† | Kasota | Seebe | CDC Earl | Harrington | Mean | Rating index |
| Abyssinian | 2.5 | 2.8 | 2.3 | 3.0 | 2.6 | 2.5 |
| AC Harper | 6.5 | 5.8 | 6.8 | 6.3 | 6.3 | 3.3 |
| AC Stacey | 9.0 | 4.5 | 3.8 | 3.5 | 5.2 | 17.3 |
| API/CM67-B//AGER | 8.3 | 6.5 | 6.5 | 5.5 | 6.7 | 8.3 |
| Atlas | 9.0 | 4.0 | 3.8 | 4.3 | 5.3 | 16.0 |
| Atlas46 | 4.5 | 2.5 | 4.0 | 3.8 | 3.7 | 6.3 |
| Atlas68 | 6.5 | 4.0 | 6.3 | 4.8 | 5.4 | 9.0 |
| CDC Dolly | 9.0 | 5.5 | 4.8 | 4.3 | 5.9 | 15.0 |
| CDC Earl | 6.3 | 6.5 | 9.0 | 8.5 | 7.6 | 10.3 |
| Duke | 5.5 | 5.5 | 7.5 | 7.5 | 6.5 | 8.0 |
| Falcon | 7.0 | 6.8 | 5.5 | 5.8 | 6.3 | 5.5 |
| Gatillo-Bar | 7.0 | 2.5 | 4.0 | 3.8 | 4.3 | 13.8 |
| Harrington | 9.0 | 9.0 | 8.8 | 8.3 | 8.8 | 2.5 |
| Jaeger | 6.8 | 4.8 | 8.8 | 7.5 | 6.9 | 12.8 |
| Johnston | 5.8 | 4.8 | 6.0 | 4.8 | 5.3 | 4.8 |
| Kasota | 8.8 | 4.3 | 3.8 | 3.5 | 5.1 | 16.3 |
| Kitchin | 4.8 | 4.0 | 4.3 | 3.8 | 4.2 | 3.3 |
| Leduc | 6.3 | 6.8 | 7.5 | 4.5 | 6.3 | 9.5 |
| Mahigan | 8.8 | 3.8 | 4.3 | 4.0 | 5.2 | 15.3 |
| Manny | 5.8 | 1.0 | 0.8 | 2.0 | 2.4 | 16.0 |
| Modoc | 6.0 | 6.5 | 8.0 | 6.5 | 6.8 | 6.0 |
| Niobe | 6.3 | 5.3 | 9.0 | 7.8 | 7.1 | 12.8 |
| Niska | 8.5 | 7.5 | 4.3 | 5.8 | 6.5 | 14.5 |
| Osiris | 3.8 | 1.5 | 3.0 | 2.5 | 2.7 | 7.3 |
| Peregrine | 7.0 | 8.0 | 7.5 | 7.3 | 7.4 | 3.3 |
| Ponoka | 8.5 | 5.0 | 3.8 | 3.8 | 5.3 | 15.5 |
| Seebe | 3.5 | 3.3 | 3.3 | 2.5 | 3.1 | 3.0 |
| Shyri | 3.8 | 3.8 | 3.5 | 3.8 | 3.7 | 0.8 |
| Trebi | 7.3 | 6.5 | 7.3 | 7.3 | 7.1 | 2.3 |
| Trochu | 7.8 | 6.5 | 8.0 | 6.5 | 7.2 | 5.8 |
| Turk | 3.3 | 2.8 | 5.8 | 4.0 | 3.9 | 9.8 |
| Vivar | 7.8 | 7.5 | 8.3 | 6.5 | 7.5 | 5.5 |
| LSD .05 | 1.1 | 1.5 | 1.3 | 1.1 | | |
| Mean pathotype severity | 6.6 | 5.0 | 5.6 | 5.1 | | |
† Hudson data not included due to winter growth habit. No symptoms occurred on Hudson with the Kasota, Seebe, and CDC Earl pathotypes, while trace to low scald levels were observed for the Harrington pathotype.
When averaged over the four separate pathotype experiments, cultivars Manny and Seebe had among the lowest scald severities, while Harrington, CDC Earl, Vivar, Peregrine, Trochu, Niobe, and Jaeger had among the highest levels of scald, while the remaining cultivars had intermediate disease levels. Of the accessions, Abyssinian, Shyri, Osiris, Atlas46 and Turk had among the lowest disease severities, while API/CM67-B//AGER, Modoc, and Trebi had the highest scald levels. A greater potential for pathotype specific responses were indicated by the ranking index for a number of cultivars with higher index values, especially for AC Stacey, CDC Dolly, Kasota, Mahigan, Manny, and Ponoka. For example, AC Stacey, Kasota, Manny, and Ponoka tended to have the highest levels of disease with the Kasota pathotype, while CDC Earl, Duke and Niobe tended to have the highest rating when inoculated with either the CDC Earl or Harrington pathotype. Accessions with the highest index values included Atlas, which tended to have the highest rating with the Kasota pathotype. Consistently low ratings over all pathotypes and lower rating index values occurred for Seebe, Shyri, Osiris, Abyssinian, and Kitchin, while consistently higher ratings and lower index values occurred for Harrington, Peregrine, and Trebi.
Results from Spearman’s rank correlations suggested that there may be an interaction between pathotype and cultivar. Confounding with location by cultivar interaction effects may be a potential concern; however, all four experiments were planted in the same experimental field and were only spaced 15 to 25 m apart. In addition, seedbed preparations, fertilizer, seeding methodologies, seeding date, and inoculation protocols were similar. Thus, cultivar differences were likely more a function of the interaction of pathotype by cultivar rather than location by cultivar. Cultivar reactions for the Kasota pathotype were not correlated (P>0.05) with those from the CDC Earl or Harrington pathotypes, while there was a significant low-moderate correlation with reactions from the Seebe pathotype (P<0.01, rs = 0.5). Reactions to the Seebe pathotype were moderately correlated with those from the CDC Earl (P<0.01, rs = 0.7) and Harrington (P<0.01, rs = 0.8) pathotypes, while reactions for the CDC Earl and Harrington pathotypes were highly correlated (P<0.01, rs = 0.9). The CDC Earl and Harrington pathotypes were collected from samples taken from a field area used for scald screening in 2001 and 1999, respectively, and by 1999, scald pathotypes virulent on CDC Earl had already occurred in this area. The Kasota pathotype was taken from the same general field in 2000, but from plots that had been in continuous Kasota for 3 years in a row, while the Seebe pathotype was taken from a screening nursery in Edmonton, AB in 2003.
The current study identified the occurrence of R. secalis pathotypes with the ability to overcome some of the most effective sources of scald resistance present in commercially available barley cultivars grown in Alberta. Of particular concern was the Kasota pathotype which produced very susceptible field reactions on AC Stacey, CDC Dolly, Kasota, Mahigan, Niska, Ponoka, API/CM67-B//AGER, and Atlas. In addition, an increased rating was observed on Manny when inoculated with the Kasota pathotype, which is in contrast with other recent trials where Manny has maintained a very good reaction to scald. The scald rating on Seebe inoculated with the Seebe pathotype was low in this trial, whereas other recent observations have shown the occurrence of susceptible-type reactions for Seebe with other scald pathotypes derived from Seebe. Further research with the Kasota and Seebe pathotypes and monitoring of Manny and Seebe reactions are underway. The CDC Earl pathotype produced very susceptible field reactions on CDC Earl, Jaeger, Modoc, Niobe, Trochu, and Vivar, while the Harrington pathotype produced susceptible reactions mainly on CDC Earl. The current study also demonstrated that Abyssinian, Atlas46, Atlas68, Gatillo-Bar, Hudson, Kitchin, Osiris, Shyri, and Turk may possess useful sources of resistance for future Alberta barley cultivars.
Differential responses within and between pathotypes indicated potential compatible barley cultivar combinations that could be used as part of short-term cultivar rotation strategies for disease management. Cultivars that may have potential to produce low to moderate levels of disease when grown in rotation with Kasota include Seebe, Duke, and Johnston, while AC Harper, CDC Earl, Leduc, and Niobe may also have some potential. Low to moderate disease levels may also be possible with Seebe in rotation with AC Stacey, Jaeger, Johnston, Mahigan, and Manny, while AC Harper, CDC Dolly, CDC Earl Duke, Niobe, Ponoka, and Trochu may also have some potential. Other cultivars that may be useful in rotations with CDC Earl include AC Stacey, CDC Dolly, Mahigan, Manny, Niska, and Ponoka.
Acknowledgements
The authors graciously acknowledge the technical assistance of Denise Orr, Noryne Rauhala, Deb Clark, and Jackie Busaan. The generous funding of the Alberta Barley Commission and Agriculture and Agri-Food Canada’s Matching Investment Initiative program is also acknowledged.
Literature Cited
Littel, R. C., Milliken, G. A., Stroup, W. W., and Wolfinger, R. D. 1996. SAS System for Mixed Models. SAS Institute, Cary NC. 656 pp.
Tekauz, A. 1991. Pathogenic variation in Rhynchosporium secalis on barley in Canada. Can. J. Plant Pathol. 13: 298-304.
Turkington, T.K., Xi, K., Tewari, J.P., Lee, H.K., Clayton, G.W., and Harker, K.N. 2005. Cultivar rotation as a strategy to reduce leaf diseases under barley monoculture. Can. J. Plant Pathol. 27: 283–290.
Xi, K., Turkington, T.K., Helm, J.H., and Bos, C. 2002. Pathogenic variation of Rhynchosporium secalis in Alberta. Can. J. Plant Pathol. 24: 176-183.
Xi, K., Turkington, T.K., Helm, J.H., Briggs, K.G. and Tewari, J.P. 2003. Distribution of Rhynchosporium secalis pathotypes and cultivar reaction on barley in Alberta. Plant Dis. 87: 391-396.
Turkington, T.K.* (1), and K. Xi (2)
*Corresponding author: turkingtonk@agr.gc.ca
(1) Lacombe Research Centre/Beaverlodge Research Farm, Agriculture and Agri-Food Canada, Lacombe, AB, T4L 1W1.
(2) Field Crop Development Centre, Alberta Agriculture, Food and Rural Development, c/o Lacombe Research Centre, Agriculture and Agri-Food Canada, Lacombe, AB, T4L 1W1
Presented at the 18th North American Barley Researchers Workshop, July 17-20, 2005 |
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